There are two problems: (a) elevator rollback and rollforward prior to start of a normal run and (b) calibration of the loadweighing system. These problems relate to operation of the elevator (a) during installation and (b) after installation, respectively.
Movement of the car prior to being commanded to run at the start of a normal run can lengthen the run time because the car must be re-leveled and brought to a standstill before going on a run. Unintended movement of the car may occur if pre-torque armature current applied to an elevator drive motor is incorrect so that the car does not stay still after the brake is lifted. This causes passenger discomfort.
Armature current is proportional to the load on the car: ##EQU1## where I.sub.ARM is the armature current;
K.sub.T is a torque constant; PA1 R is the length of the torque arm; PA1 LW is the load weight, the force tangent to the sheave which may be expressed as % LOAD (the weight in the car as a percentage of full load) minus % OVERBALANCE; and PA1 T is the torque.
The two problems are as follows:
(1) At installation, the drive must be adjusted to provide an armature current during pre-torque (bias current) to keep the car from moving when the brake is lifted prior to a run. A parameter MBIAS scales bias torque based on the overbalance, in the car (that is, when the car is carrying full load, the motor is carrying full load minus the overbalance); the overbalance is the portion of the counterweight greater than the weight of the car (% OVERBALANCE). The drive receives loadweighing information from the car controller, formatted as a percentage offset from the weight of a balanced car; thus, empty car load is zero minus overbalance. Thus, MBIAS and % OVERBALANCE must be properly adjusted at installation to give accurate pre-torque armature currents. A method to quickly and accurately set these parameters is needed. Presently, these numbers are entered from a table, with MBIAS being adjusted in an imprecise manner at installation to give approximately the right pre-torque value, usually based on load in the car. ##EQU2##
(2) After installation and during the life of an elevator, loadweighing must be periodically re-adjusted to keep the pre-torque current accurate enough to prevent unintended motion of the car after the brake is lifted. This expensive procedure requires the transport of heavy weight carts to and from the job site to recalibrate the loadweighing gain and offset in the controller. The weights in the weight carts are used as the recalibration standard. Some better method of compensating for drift in the loadweighing system is needed.